To be pedantic (but in order to be specific), z64 is the software renderer from MAME (ported to plugin specs by ziggy) and z64gl is ziggy's OpenGL version of it.

I do like the idea of having a software-rendering LLE plugin with an option to use client-side hardware rendering (defined by the OpenGL driver) to switch between software and accelerated graphical rendering mode.

I even like the idea of that same plugin having HLE added to it. With LLE you can choose to use the hardware-accelerated client-side rendering rules over per-pixel-forced software rendering (default for accuracy) while with HLE it is just for speed, so in HLE it is the 3-D OpenGL renderer always, which is based on the implementation details of the LLE version's accuracy.

But what you're proposing sounds like the same thing going from the opposite direction. A good HLE graphical renderer should be based on the details of an LLE accurate renderer, not the other way around. So unless you can somehow convert the HLE in PJGlide64 to an LLE version for users of LLE (with some of the same flaws inherent within the HLE design's information), it sounds like the only thing you can do is use another LLE codebase entirely such as angrylion's. Again, putting 2 DLLs into one unnecessarily.